Wax-flux composition containing a diester of sulfomalic acid for soldering

ABSTRACT

Wax-flux compositions for use in low-temperature soldering processes comprise (A) a major amount of a wax and (B) an effective amount to cause fluxing, at or below the soldering temperature of the piece to be soldered, of a wax-soluble diester of sulfomalic acid. A process for soldering electrical components to a printed circuit board comprises (A) applying a molten wax-flux composition as described above to the metal pattern side of the board, (B) allowing the wax-flux composition to solidify, (C) trimming the components leads, and (D) soldering the components to the printed circuit board.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to low-temperature soldering processes. Thisinvention also relates to wax-flux compositions useful in suchlow-temperature soldering processes.

A commercially popular labor saving process for assembling electroniccomponents is to install these components on a printed circuit board.After most, if not all, of the components are installed on the circuitboard, the components are soldered into place by a process known as wavesoldering.

One difficulty with the wave soldering process is that the componentshave leads which extend 1-2 inches beyond the surface of the printedcircuit board. This long length requires a very high standing wave ofsolder. The standing wave can be forced high enough to solder these longlead components but this has disadvantages. Among them, a large amountof solder is lost in coating all these long leads and large globs ofsolder form between groups of leads which extend below the board.However, by soldering the printed circuit board prior to lead trimming,the components are fixed in place and can be trimmed by mechanical meansrather than being hand trimmed, although hand-trimming, of course, isquite feasible.

The disadvantage of trimming a circuit board after it has been solderedis that critical applications, such as military or space applicationsrequire the circuit boards be resoldered subsequent to the trimming.This, of course, greatly increases the expense to the party doing thesoldering as well as increases the risk of damage to the components.

An alternative to soldering the long leads is to hand trim them and bendthe stubs over against the board before soldering. The bent-over stubshold the components in place during the soldering operation. Advantagesof this method are that only one soldering operation is required andonly a low wave of solder need be maintained. The disadvantage is thelarge amount of hand labor required by this method.

What is needed is an inexpensive method which does not involve solder tofix the components in place on the circuit board while the leads arebeing trimmed and which does not interfere with the subsequent solderingoperation.

Wave soldering also uses a flux to clean the printed circuit board andprepare both the component leads and the printed circuit board foil toreceive the solder. In a typical commercial installation, a liquid fluxis maintained in a bath which contains fritted aspirators. Aspiratingthe liquid results in a foam head on the flux tank. The amount ofaspiration is controlled so that the foam head just touches the printedcircuit boards conveyed above it. A light film of the liquid flux isthen deposited on the printed circuit board which subsequently passesover a heater. This preheats the circuit board. The printed circuitboard immediately thereafter passes over the standing wave of solder andthe soldering process is accomplished.

One disadvantage of using this method of applying the soldering flux tothe parts which are to be soldered is that extra equipment is needed. Inaddition, the flux must be aspirated very carefully and constantattention must be paid to the machinery to see that the flux headremains at the right height. If the flux head is too low, obviously theprinted circuit boards will not be properly treated and a poor solderingjob will result. If the flux head is too high, the entire circuit boardincluding the components may be coated with flux which requiresadditional cleaning as well as wastes soldering flux. What is needed isa convenient way to apply the soldering flux to the printed circuitboard which does not require special equipment and special attention.

The present invention not only solves the problem of how to firmly fixthe components to the printed circuit board for the lead-trimmingoperation, but also the problem of how to conveniently apply solderingflux to the printed circuit board all in a single-step operation.

SUMMARY OF THE INVENTION

A wax-flux composition for use in low-temperature soldering processes isprovided comprising (A) a major amount of a wax and (B) an amounteffective to cause fluxing, at or below the soldering temperature of thepiece to be soldered, of a wax-soluble diester of sulfomalic acid. Thehardness of the wax at room temperature is chosen to effectively holdthe electronic components in place during an automatic lead-trimmingoperation. The diester of sulfomalic acid provides fluxing action duringthe subsequent soldering step thereby obviating the need to separatelyapply a soldering flux.

DETAILED DESCRIPTION OF THE INVENTION

Wax-flux compositions for use in a low-temperature soldering process areprovided comprising: (A) a major amount of a wax having an AmericanMelting Point (AMP determined by ASTM D 127-60) between 40°C and 100°Cand (B) an amount effective to cause fluxing, at or below the solderingtemperature of the piece to be soldered in said process, of awax-soluble diester of sulfomalic acid. In a second embodiment of theinvention a process is provided for soldering electrical components to aprinted circuit board, said board comprising a non-conducting substrateand a pattern of an electrically conductive metal laminated to saidsubstrate, said metal being solderable with a low-melting-point alloysolder, said board having component lead receiving holes through bothsaid substrate and said metal pattern, comprising:

a. applying a molten wax-flux composition described above to the metalpattern side of said board and to said component leads;

b. allowing said wax-flux composition to solidify;

c. trimming said component leads; and

d. soldering said component to said printed circuit board with alow-melting-point alloy solder.

THE WAX

Suitable waxes for use in the wax-flux compositions of this inventionand in the soldering processes of this invention include any wax or waxblend which is solid at room temperature and has a melting point belowthe temperature at which the soldering process will be conducted.Preferably the waxes have an AMP of at least 40°C and generally thewaxes will have an AMP not greater than 100°C. Preferably the waxes havean AMP between 50°C and 80°C.

Suitable waxes include petroleum-derived waxes such as the well knownparaffin waxes, microcrystalline waxes, slack waxes, scale waxes,petrolatum, etc. These waxes are obtained from the processing of crudepetroleum and are generally substantially saturated, substantiallystraight long-chain aliphatic hydrocarbons. Petroleum waxes suitable foruse in this invention have AMP's within the range specified above.

Suitable wax blends for use in this invention include the hot meltcoatings which consist of blends of petroleum waxes and polymers,copolymers or resins. Suitable materials which may be blended with thepetroleum waxes include polymers of low molecular weight olefins, suchas polymers of ethylene, propylene, butylene, isobutylene, and the like.Suitable polymers will have molecular weights from about 1,000 to about1,000,000, more usually from about 1,000 to about 50,000. These areaverage molecular weights and generally a major portion of the moleculesof the polymer will have molecular weights close to the average.

Suitable copolymers include copolymers of olefins with olefinic esterssuch as ethylene-vinyl acetate copolymers. These copolymers arecommercially available from E. I. DuPont DeNemours & Company under thetrade name ELVAX. Other suitable copolymers include copolyers ofdifferent olefins such as the copolymers of propene and butene.Typically such a copolymer will contain from about 15 to about 85 molpercent propene, more usually from about 25 to 75 mol percent propene.Typical copolymer molecular weights will range from about 1,000 to about1,000,000, more usually from about 1,000 to about 300,000.

Other suitable wax blends include wax compositions incorporatingcellulose esters or cellulose ethers. Suitable cellulose esters includealkyl esters of cellulose wherein the cellulose molecule contains, onthe average, 3 alkyl radicals per glucose unit, i.e., the celluloseesters are triesters of cellulose. Typically, the alkyl radicals containfrom about 7 to about 16 carbon atoms which include cellulosetriheptanoate, cellulose trioctanoate, cellulose tridecanoate, cellulosetrilaurate, etc.

Suitable cellulose ethers include the di- and triethers of cellulosewherein the ether radicals are hydrocarbon radicals, preferably alkylradicals each having from 1 to 18 carbon atoms, with the combined totalnumber of carbon atoms preferably being at least 12. In other words,although each of the hydrocarbon ether radicals on each glucose unit ofthe cellulose has from 1 to 18 carbon atoms, the total number of carbonatoms preferably is at least 12. In the case of diethers, one of theether radicals preferably contains at least 8 carbon atoms. Suitableethers of cellulose include cellulose methyl octyl ether, celluloseethyl octyl ether, cellulose ethyl decyl ether, cellulose ethyl dodecylether, cellulose ethyl tetradecyl ether, cellulose propyl octyl ether,cellulose butyl octyl decyl ether, cellulose methyl butyl amyl ether,cellulose tri(butyl) ether, cellulose methyl octyl octadecyl ether, etc.

Typically the cellulose, prior to etherification or esterification, hasa molecular weight ranging from about 40,000 to about 500,000. Typicallythe cellulose esters and cellulose ethers are used in the waxcompositions in amounts ranging from about 2 to about 30 percent,preferably from about 10 to about 20 percent by weight.

Suitable waxes for this invention also include waxes obtained fromnatural sources, such as animal, vegetable or insect sources. Suitablewaxes include beeswax, carnuba wax, montan wax, wool wax, and the like.

Another type of wax suitable for use in this invention includes the wellknown Fischer-Tropsch waxes. Fischer-Tropsch waxes are waxes synthesizedby the familiar Fischer-Tropsch process. By this process, coal is burnedin the presence of oxygen and steam to produce hydrogen and carbonmonoxide, which are then reacted in the presence of catalyst to make thedesired hydrocarbon wax. Suitable Fischer-Tropsch waxes for thisinvention can be obtained under the trade name "Paraflint". Theseparticular Fischer-Tropsch waxes have a high molecular weight, on theaverage in the range of about 750 to 1000 and generally consistessentially of straight-chained hydrocarbons.

Although the above waxes have been mentioned individually, suitablewaxes for this invention include mixtures of various proportions of theabove-mentioned waxes.

THE FLUXING AGENT

The wax-flux compositions provided by this invention and useful in theprocess of this invention include a wax-soluble diester of sulfomalicacid. Suitable wax-solublizing esterifying substituents contain fromabout 1 to about 22 carbon atoms such that the total number of carbonatoms contained in the ester substituents is at least 12 and up to about30 carbon atoms.

Preferably the diesters of sulfomalic acid useful in this invention havethe following general formula: ##EQU1## wherein each R independentlyrepresents a wax-solublizing group such as alkyl containing 1-22 carbonatoms, such as methyl, ethyl, propyl, butyl, isobutyl, tert-butyl,pentyl, hexyl 2-methylhexyl, octyl, 2-ethylhexyl, tripropenyl, decyl,dodecyl, tetrapropylenyl, tridecyl, tetradecyl, pentadecyl,pentapropylenyl, hexadecyl, octadecyl, hexapropylenyl, eicosyl, docosyl,and the like such that the total number of carbon atoms contained in allof said R groups is from 12 to about 30 carbon atoms. Preferably each Rindependently represents an alkyl containing 4 to 22 carbon atoms andmore preferably 8 to 22 carbon atoms. For ease of manufacture, the Rgroups preferably are all the same alkyl group.

The diester having a free (i.e., not in salt form) sulfonic acid groupmay be prepared directly. The acids are prepared by esterification ofsulfomalic acid with a suitable alcohol or mixture of alcohols. Examplesof suitable alcohols include the alcohol form of the alkyl radicalsmentioned above for the value of R.

Esterification is easily accomplished by heating a quantity of at least2 mols (preferably a slight excess) of the appropriate alcohol with onemol of sulfomalic anhydride or acid for a period of 10 minutes to 1 hourat temperatures in the range 50°C to 150°C.

The sulfomalic acid starting material may be produced from diethylsulfothiosuccinate by reaction with HCl in aqueous solution. Thisprocess is disclosed in the Journal of the Russian Physical ChemistrySociety, as noted in Beilstein, Handbook of Organic Chemistry, Vol. 4,1st Supp., p. 314. Alternatively, sulfomaleic anhydride can behydrolyzed to sulfomaleic acid and then hydrated to sulfomalic acid. Thesulfomaleic anhydride starting material may be produced conventionallyby reacting maleic anhydride with a slight molar excess of sulfurtrioxide, heating for 1-2 hours at 80°-125°C. The excess sulfur trioxidemay be stripped off and recovered from the mixture.

THE WAX-FLUX COMPOSITIONS

The wax-flux compositions of this invention are prepared by melting thewax or wax blend and dissolving the diester therein. The mixture is thenstirred until homogeneity is obtained. The wax-flux composition is castinto blocks or slabs and allowed to solidify unless it is to beimmediately used.

The wax-flux compositions of this invention will contain an amounteffective to cause fluxing, at or below the soldering temperature of thepiece to be soldered, of the wax-soluble diester. This effective amountcan readily be determined by those skilled in the art by a few simplesoldering tests to determine the minimum amount necessary. Any amountabove the minimum necessary to cause effective fluxing generally is notnecessary and increases the cost of the wax-flux composition withoutadditional benefits during the soldering process. Typically, thewax-flux compositions will contain from about 1 to about 20 weightpercent of the diester, more usually from about 3 to about 12 weightpercent and preferably from about 5 to about 10 weight percent.

THE PROCESS

The wax-flux compositions of this invention are useful in the solderingprocess of this invention. The soldering process for this inventionprovides a method for soldering electrical components to a printedcircuit board. Printed circuit boards are well known in the art. Theyconsist of a non-conducting substrate and a pattern of an electricallyconductive metal laminated to the substrate. Typically, the substrate isa phenolic resin or an epoxy fiberglass composition board. Almostuniversally the electrically conductive metal is a copper foil. Thepattern of the conductive metal on the substrate can be prepared by anumber of means well known to those skilled in the art.

The printed circuit board has lead receiving holes. The electricalcomponent leads are inserted through the holes from the non-pattern sideof the board. Typically, the electrical components have leads whichextend 1 to 2 inches beyond the pattern side of the board. In some priorart methods of soldering the components to the board, the leads aresoldered to the metal pattern and then the lead is trimmed relativelyclose to the board.

The process of this invention is particularly applicable to automatedsoldering with wave soldering machines. Wave soldering machines arecommercially available from a number of sources and are used by most ofthe electronics industry manufacturers. As mentioned above, aparticularly troublesome problem in trying to solder electroniccomponents to printed circuit boards with wave soldering machines is thelong leads of the electronic components which extend beyond the board.

In the process of this invention, the component leads are insertedthrough the holes of the printed circuit board. Then without trimmingthe leads, the molten wax-flux composition is applied to the metalpattern side of the circuit board and allowed to solidify. Once thewax-flux composition has solidified, the components are firmly attachedto the board and the component leads can be trimmed prior to thesoldering operation.

The component leads of the waxed board can be trimmed by any availablemethod including hand-trimming and automated trimming. A most convenientmethod of trimming uses a high-speed circular metal-cutting saw. Oneexample of such a saw is a 16-inch high-speed circular saw sold byHollis Engineering Company of Nasha, N.H. under the name Holli-Cutter.To use the Holli-Cutter, the printed circuit board is placed on aconveyor which passes over the circular saw. The space between theprinted circuit board and the saw blade is adjusted to give trimmedleads of the desired length.

After the leads are trimmed, the electrical components are soldered tothe printed circuit board with a low-melting-point alloy solder. Thissoldering operation can be conducted by any of the conventionallyavailable means such as by hand-soldering or by automated wavesoldering.

In a preferred embodiment of the process, the wax-flux composition isapplied to the printed circuit board by contacting the metal patternside of the board with the crest of a standing wave of molten wax-fluxcomposition. Thereafter the wax-flux composition is allowed to solidify,generally with the aid of a forced draft of cold air. The componentleads are then trimmed by an automated cutting means which is placed inline with the conveyor carrying the printed circuit board. Thereafterthe components are soldered to the board by contacting the metal patternside of the board with the crest of a standing wave of thelow-melting-point alloy solder, typically, in one of the commerciallyavailable automated wave soldering machines.

When a wave soldering machine is used, it is often found advantageous tomaintain a thin film of tinning oil over the surface of the standingpool of molten solder in the catch basin of the wave soldering machine.The tinning oil helps reduce oxidation and dross formation of the moltensolder which can lead to poor soldering. Suitable tinning oils areavailable commercially. One such tinning oil is available from HollisEngineering Company and comprises a bright stock containing fats, fattyacids, naturally occurring unsaturated long-chain acids, and oxidationinhibitors.

Typically, the tinning oil is changed approximately every 8 hours. Inthe process of this invention, the tinning oil becomes contaminated withthe wax-flux composition which melts and is removed from the printedcircuit board during the soldering operation. It has been found such asmall quantity of wax-flux composition is coated onto each board, thateven with a very high soldering rate, the wax-flux compositioncontamination of the tinning oil does not cause a significant reductionin the useful life of the oil. This slight increase in the frequency ofchanging the tinning oil is a very small price to pay for theconvenience and improved soldering offered by the process of thisinvention.

Subsequent to the soldering process, any remaining traces of the fluxare generally removed from the printed circuit board. This preventspotential long-term corrosion of the printed circuit board and thecomponent leads.

The flux and tinning oil can be removed either by vapor degreasing oraqueous washing. Machines are available commercially for practicing bothmethods. For aqueous washing, these generally resemble largedishwashers. Typically, the wash water is maintained between 60°C and72°C. Generally a detergent is used during the washing process. Typicaldetergents are mixtures of alkaline surface-active agents and nonionicsurface active agents. Suitable commerically available detergentsinclude Aqua-Clean available from Hollis Engineering Company andLonco-Terge available from Lonco Corporation.

As a final step of the washing process the circuit boards are rinsedwith warm or hot deionized or distilled water to remove the last tracesof the fluxing agent and the detergents.

EXAMPLES

The following examples illustrate the preparation of diesters useful inthe wax-flux compositions of the invention. The examples areillustrative and non-limiting.

EXAMPLE 1 Esterification of Sulfomalic Acid with 1-Octanol

1.8030 g. of 68.5% aqueous sulfomalic acid plus 1.6602 g. of 1-octanol(about 2.2 mol ratio) were mixed together in a small vial in a 110°C oilbath for thirty minutes giving 2.9886 g. residue and then heated andstirred thirty more minutes with a gentle N₂ stream sweeping the airspace giving 2.6000 g. of a homogeneous viscous fluid. Analysis by longchain quaternary ammonium titration ["Analysis of Synthetic AnionicDetergent Compositions," R. House and J. L. Darragh, Anal. Chem. 26(1954) 1492] showed 92% mol percent yield of titratable sulfonic acidbased on the sum of the sulfomalic acid and sulfomaleic acid in thestarting sample. Titration with standard base showed 1.90milliequivalents of original acidity were lost per meq. of activeproduced.

EXAMPLE 2 Esterification of Sulfomalic Acid with n-Decanol

3.40 g. of 19.2% aqueous sulfomalic acid was mixed with 1.03 g. ofn-decanol (about 2.1 mol ratio) and a few ml. of acetone added to give ahomogeneous mixture. This was then evaporated on a 30°C surface withstirring and with a gentle N₂ stream sweeping the air space yielding1.68 g. of a clear viscous fluid. Analysis for titratable sulfonic acidshowed 95 mol percent yield. Titration with standard base showed loss of1.99 meq. of original acidity per meq. of active produced.

The following example illustrates soldering with the compositions of theinvention.

Wax-flux compositions of this invention are tested in commerciallyavailable wave soldering equipment. The results of these tests arecompared against soldering operations conducted in commerciallyavailable wave soldering equipment using a commercially available wavesoldering flux, namely Alpha Reliafoam 809 Flux available from AlphaMetals, Inc., 56 Water Street, Jersey City, N.J. 07304. This flux is oneof the most widely used commercially available liquid fluxes for wavesoldering. It has been found to have the capacity to satisfactorily fluxcopper printed circuit boards having visible dirt, tarnish andcorrosion.

For soldering sample printed circuit boards using the commerciallyavailable Alpha flux, the wave soldering equipment comprises an in-lineunit having a foam flux applicator, a preheater, and a wave solderapplicator. The in-line unit includes a conveyor which transports theboards through the various steps of the soldering operation. Printedcircuit boards for the soldering operation are prepared by inserting theleads of electronic components through the holes in the printed circuitboard, trimming the leads, and bending the stubs over against the copperfoil. The printed circuit boards used in these tests are deliberatelyprepared to be dirty, tarnished and difficult to solder.

For soldering printed circuit boards using the waxflux composition ofthis invention the wave soldering equipment includes a wax-fluxapplicator in which the molten wax-flux composition at about 75°-95°C ispumped into a standing wave about 4-5 cm in height. Next in line is alead trimmer which is, in this case, a Holli-Cutter available fromHollis Engineering, Inc. The Holli-Cutter comprises a 16-inch high-speedtungsten carbide-tipped circular saw. After the lead trimmer is theboard preheater and then the wave solder applicator. The leads ofelectronic components are inserted through the holes in the printedcircuit board. The leads are left untrimmed. The board with theelectronic components is placed on the conveyer of the solderingapparatus. The printed circuit board is conveyed first over the wax-fluxwave. A thin film of wax-flux remains on the underside of the printedcircuit board. The board then passes over a forced draft of cool airwhich cools and solidifies the wax. The board then passes over the leadtrimmer and the electronic component leads are trimmed to the desiredlength. The board then passes over the flux preheater and over the waveof solder. The printed circuit boards soldered with the wax-fluxcomposition of this invention and according to the process of thisinvention are then compared with the printed circuit boards solderedusing the commercially available liquid flux.

EXAMPLE 3

The wax-flux composition comprises 90% weight refined base wax having anAMP of 154/156 and 10% of the dioctyl ester of sulfomalic acid. Thewax-flux composition securely attaches the electronic components to theprinted circuit board and allows fully satisfactory automated leadtrimming. During the soldering step this wax-flux composition providesfluxing and allows soldering comparable to the commercially availablesoldering flux. A certain amount of dewetting is noted when thesoldering wave contains no tinning oil.

Subsequent to the soldering step the printed circuit board is washed ina circuit board washer using a 71°C aqueous detergent solutioncontaining Hollis Aqua-Clean. The washed boards are completely clean andfree of any wax or oil residue.

What is claimed is:
 1. A wax-flux composition for use in alow-temperature soldering process, comprising:A. a major amount of a waxhaving an average melting point between 40°C and 100°C, and B. an amounteffective to cause fluxing, at or below the soldering temperature, ofthe piece to be soldered in said process of a wax-soluble alkyl ester ofsulfomalic acid.
 2. A wax-flux composition of claim 1 wherein saidwax-soluble ester has the following formula: ##EQU2## wherein each Rindependently contains from 1 to 22 carbon atoms such that the sum ofthe carbon atoms contained in both R groups is at least
 12. 3. Awax-soluble composition of claim 2 wherein said R groups contain from 4to 22 carbon atoms.
 4. A wax-flux composition of claim 2 wherein said Rgroups contain from 8 to 22 carbon atoms.
 5. A wax-flux composition ofclaim 4 wherein said ester comprises 1 to 20 weight percent of saidcomposition.
 6. A wax-flux composition of claim 4 wherein said estercomprises 3 to 12 weight percent of said composition.
 7. A wax-fluxcomposition of claim 5 wherein R represents octyl and said wax has anaverage melting point of from 50°C to 75°C.
 8. A wax-flux composition ofclaim 5 wherein R represents undecyl and said wax has an average meltingpoint of from 50°C to 75°C.
 9. A wax-flux composition of claim 5 whereinR represents dodecyl and said wax has an average melting point of from50°C to 75°C.
 10. A process for soldering an electrical component to aprinted circuit board, said board comprising a non-conducting substrateand a pattern of an electrically conductive metal laminated to saidsubstrate, said metal being solderable with a low-melting-point alloysolder, said board having component lead receiving holes through bothsaid substrate and said metal pattern, said component having electricalleads, and said component being mounted on said board by having at leastone of said leads inserted through a said lead receiving hole, saidprocess comprising:a. applying a molten wax-flux composition of claim 1to the metal pattern side of said board and to said component lead; b.allowing said wax-flux composition to solidify; c. trimming saidcomponent lead; and d. soldering said component to said printed circuitboard with a low-melting-point alloy solder.
 11. The process of claim 10wherein:a. said wax-flux composition is applied to said board bycontacting the metal pattern side of said board with the crest of astanding wave of said molten wax-flux composition; and b. said componentis soldered to said board by contacting the metal pattern side of saidboard with the crest of a standing wave of said low-melting-point alloysolder.
 12. The process of claim 11 wherein said component lead istrimmed by a circular saw.